Beam systems, such as electron beam systems, ion beam systems, laser beam systems, cluster beam system, and neutral particle beam systems, are used to create features on a surface by etching or deposition. Beam-induced deposition processes use a precursor gas that reacts in the presence of the beam to deposit material on the surface in areas where the beam impacts. For example, a gaseous organometallic compound, such as tungsten hexcarbonyl, is provided near the sample and is adsorbed onto the surface. The organometallic compound decomposes in the presence of a charged particle beam, such as an ion beam or an electron beam, to form a metal that remains on the surface and a volatile organic compound that is removed by a vacuum pump. Etching processes use a precursor gas that reacts with the surface of the work piece to form a volatile compound. For example, iodine can be used to etch a silicon wafer. The iodine reacts in the presence of the beam to form a volatile silicon iodine compound, which leaves the sample surface and is removed by the vacuum pump.
Precursor gases are introduced into the vacuum by a “gas injection system” or “GIS.” Gas injection systems typically include a gas source and a gas director, such as a needle or funnel, that is positioned near the sample and directs the gas toward the work piece. A precursor gas that is generated from a material that is solid or liquid at room temperature is typically supplied from a crucible within the vacuum chamber. The flow of gas is generated by heating the solid or liquid to increase its vapor pressure, causing gas to flow through the gas director and into the vacuum chamber. For example, tungsten hexacarbonyl is a solid at room temperature and is typically heated to about 55° C. or 60° C. to raise its vapor pressure to cause a suitable flow into a vacuum chamber.
One prior art system is described, for example in, U.S. Pat. No. 5,435,850 to Jorgen Rasmussen for a “Gas Injection System.” The gas injection system of Rasmussen includes a crucible in which a solid or liquid source material is stored. The crucible is positioned within the vacuum chamber. The crucible is heated to increase the vapor pressure of the source material, and the gas from the source material then flows to the sample. The gas flow is regulated by the amount of heat supplied to the crucible and by positioning a plunger within a valve to control the size of the valve opening. The limited crucible capacity requires frequent refilling of the crucible in many applications. The dangerous nature of some of the precursor chemicals necessitates special safety equipment during refilling, which equipment may not be readily available in the field. Such systems also require realignment after each refill so that the needle is pointing toward the impact point of the charged particle beam. Controlling the temperature and the valve opening provides a limited ability to control the pressure within the sample chamber of the charged particle beam system.
Another type of gas injection system is described in U.S. Pat. No. 5,851,413 to Casella for a “Gas Delivery Systems for Particle Beam Processing.” In the systems of Casella, the precursor is stored outside the vacuum chamber, and flows through a conduit into a gas concentrator near the sample. Systems that store the precursor gas outside the vacuum chamber typically include a valve, such as a stepper-motor-controlled diaphragm valve, to control the gas flow.
Control of pressure of the precursor gas in the charged particle beam sample chamber is limited in prior art systems because the pressure is controlled only by the controlling the temperature of the gas source or the degree of opening of the diaphragm valve. A vacuum pump is continually removing gas from the sample chamber, and so the pressure in the equilibrium chamber is reached when the gas flow in is equal to the gas flow out. Apertures are required in the input gas path to restrict the flow so that a low pressure can be maintained in the sample chamber. Such apertures, however, increase the time required to bring the sample chamber up to the desired operating pressure.
FIG. 1 shows a typical prior art diaphragm valve 100 used in a gas injection system. Valve 100 includes a valve body 102 and an actuator, such as a stepper motor 104 or a pneumatic valve that controls the position of a valve stem 106 that positions a diaphragm 108 over a seat 110 at an opening in a supply pipe 112. When the valve stem 106 presses the diaphragm against the opening in seat 110, no gas flows through the opening. When the valve stem moves away from the opening, as shown in FIG. 1, the diaphragm allows gas to flow into the valve from supply pipe 112 and out of the valve through outlet pipe 114. Pressure is measured at exit of gas system and then correlated using a calibration table to feed back to control system.
The diaphragm valve is operated in a partially open position, with the flow dependent on the degree of opening and the upstream gas pressure. Once the degree of opening that provides the desired chamber pressure is determined, the valve typically remains in that position, unless the pressure in the chamber needs adjustment. The flow control is coarse and the position of the valve stem is not highly correlated to a flow rate. That is, a valve setting that provides a first sample pressure chamber on the first system will not necessarily provide the same pressure on a second system. In such processes, it is preferably to maintain a desired ratio between process gases. It can be difficult to maintain the desired ratio when control of each component is inexact.
US Pat. Pub. No. 2009/0223451 describes a system for delivering precursor gases to a beam instrument. The system uses a carrier gas to dilute and carry the precursor gases from one or more crucibles though a single main line to a needle and into the sample vacuum chamber. Flow of the carrier gas and the gas from each crucible is controlled in part by controlling the duty cycle of a pneumatic valve. Part of each crucible and the main line are in a gas envelope that opens to the sample vacuum chamber. Use of a single main line leaves precursor gas in the main line when the crucible valve is closed, thereby requiring a purging procedure for the main line, which takes time and wastes precursor gas.